A ceramic nanocomposite is a solid material consisting of multiple ceramic phases where the phases have dimensions of less than 100 nanometers (nm).
Nanocomposite materials can have properties not seen in their microcystalline counterparts and/or enhance/improve functional properties, such as mechanical strength, fracture toughness, optical transmission, optical absorption, corrosion resistance, thermal transport resistance, electrical conductivity, etc.
In general, these properties are improved or unveiled as the nanostructure of the material is refined. Bulk materials having improved or new properties from nano-scale microstructures can be used in numerous applications, including optical and detector devices, cathodes, high strength oxide and metals, nanocomposite magnets, superconductors, and thermoelectrics.
Properties of bulk materials with nano-size grains, including nano-ceramics, differ from properties of conventional bulk materials. Dense network of interfaces or grain boundaries between nano-grains increases hardness and strength, decreases thermal conductivity, etc. Light doesn't scatter on interfaces because of very small grain size which improve transparency of polycrystalline ceramics, like alumina, magnesium aluminate spinel (MgAl2O4). For example, it was shown that nano-spinel (MgAl2O4) ceramics with grain size of about 30 nm demonstrates 40% increase of hardness in comparison with the best micron and submicron grain spinel ceramics. See J. A. Wollmershauser, B. N. Feigelson, E. P. Gorzkowski, C. T. Ellis, R. Goswami, S. B. Qadri, J. G. Tischler, F. J. Kub, and R. K. Everett, “An extended hardness limit in bulk nanoceramics,” Acta Materialia, 2014. 69(0): pp. 9-16.
Currently, bulk monolithic polycrystalline solids which have nano-scale elements in their microstructure are produced by a variety of processing approaches. These approaches include severe plastic deformation, solid state nano-precipitation, rapid solidification, consolidation/organization of colloidal crystals, sintering (including spark plasma sintering, hot pressing, hot isostatic pressing). See R. Z. Valiev, R. K. Islamgaliev, and I. V. Alexandrov, “Bulk nanostructured materials from severe plastic deformation,” Progress in Materials Science (2000) 45, pp. 103-189; A. Manaf, R. A. Buckley, and H. A. Davies, “New Nanocrystalline High-remanence Nd—Fe—B Alloys by Rapid Solidification,” Journal of Magnetism and Magnetic Materials (1993) 128, pp. 302-306; L. Q. Xing, J. Eckert, W. Loser, and L. Schultz, “High-strength materials produced by precipitation of icosahedral quasicrystals in bulk Zr—Ti—Cu—Ni—Al amorphous alloys,” Applied Physics Letters (1999) 74, pp. 664-666; C. B. Murray, S. H. Sun, W. Gaschler, H. Doyle, T. A. Betley, and C. R. Kagan, “Colloidal synthesis of nanocrystals and nanocrystal superlattices,” IBM Journal of Research and Development (2001) 45, pp. 47-56; F. Maglia, I. G. Tredici, and U. Anselme-Tamburini, “Densification and properties of bulk nanocrystalline functional ceramics with grain size below 50 nm,” Journal of the European Ceramic Society (2013) 33 pp. 1045-1066; J. E. Carsley, A. Fisher, W. W. Milligan, and E. C. Aifantis, “Mechanical behavior of a bulk nanostructured iron alloy,” Metallurgical And Materials Transactions A (1998) 29, pp. 2261-2271; Z. Zhang, F. Zhou, and E. J. Lavernia, “On the analysis of grain size in bulk nanocrystalline materials via X-ray diffraction,” Metallurgical And Materials Transactions A (2003) 34A, pp. 1349-1355; M. J. Mayo, “Processing of nanocrystalline ceramics from ultrafine particles,” International Materials Reviews (1996) 41, pp. 85-115; B. Poudel, Q. Hao, Y. Ma, Y. C. Lan, A. Minnich, B. Yu, X. A. Yan, D. Z. Wang, A. Muto, D. Vashaee, X. Y. Chen, J. M. Liu, M. S. Dresselhaus, G. Chen, and Z. F. Ren, “High-thermoelectric performance of nanostructured bismuth antimony telluride bulk alloys,” Science (2008) 320, pp. 634-638; R. Chaim, Z. Y. Shen, and M. Nygren, “Transparent nanocrystalline MgO by rapid and low-temperature spark plasma sintering,” Journal of Materials Research (2004) 19, pp. 2527-2531; and S. Paris, E. Gaffet, F. Bernard, and Z. A. Munir, “Spark plasma synthesis from mechanically activated powders: a versatile route for producing dense nanostructured iron aluminides,” Scripta Materialia (2004) 50, pp. 691-696.
However, each of these known processing approaches has significant drawbacks. Some are limited to a narrow class of materials and narrow design of nanostructure. Other approaches retain porosity in the bulk material or incorporate foreign substances in the bulk material, while others dramatically change the length scale of the nanostructure or are limited in the achievable geometry of the fabricated material.
Severe plastic deformation involves using large strains and complex stress states to produce a material with a high defect density and an ultrafine or, for some materials, nanocrystalline grain structure. Because of the use of large strains the processing approach is most often limited to single phase metals and alloys. Additionally, nanocrystalline microstructures cannot be obtained in all metal materials and when achievable, the length scale is >50 nanometers. See Valiev, supra.
Solid state precipitation produces a multi-phase nanostructure by forming and growing a new phase within a parent phase to the size of nanoscale precipitates which are homogeneously distributed inside the parent phase. This is most often achieved by increasing the temperature of the material to a temperature where the second phase is stable to form nuclei of new phase, and then relatively fast cooling rates are used to stop growth the new phase and retain the nanostructure. Such a processing approach is limited to narrow range of material systems because it requires solubility of one material in another. Additionally, forming microstructures comprised primary of nanostructures is not possible in all systems because the size of the precipitate is fundamentally controlled by intrinsic diffusion rates and kinetics and modifications to processing parameters, such as cooling rate or anneal temperature (if system is super cooled), may not allow sufficient microstructural control. See Xing, supra.
Rapid solidification forms nano-crystalline and nano-quasicrystalline materials (mixture with amorphous or larger grain material) from very fast cooling of liquid. The microstructure length scale is determined by the nucleation and growth kinetics during the short timescale of the solidification process. Generally, cooling rates on the order of ˜100,000 K/s can result in microstructures with length scales less than 1 micrometer. By increasing the solidification rate finer microstructure is obtained. However, the required fast cooling to produce nanocrystalline materials is most often only achievable when forming thin ribbon form factors. Therefore, bulk three-dimensional materials, such as those used in structural applications, are not possible. See Manaf, supra.
Colloidal crystals are ordered arrays of colloid particles, and when comprised of nano-sized colloid particles can be classified as a bulk nanomaterial. In general, the colloid particles can range in size from a few nanometers to micrometers and can be created from solution precipitation as single phase or multi-phase core-shell structures. The bulk properties of a colloidal crystal depend on the composition and size of the colloidal particle, as well as their arrangement/packing and degree of long range order. However, colloidal particles often have organic ligand and, therefore, the properties of colloidal crystal are also governed by these impurities, or if removed, porosity between the colloidal particles. See Murray, supra.
Sintering is a process of forming bulk materials from powder precursors through the application of heat and/or pressure. The process can be used with metals, ceramics, plastics, semiconductors, and other materials, where the powder precursors can range in size from nanometers to micrometers and can be single phase, two phase mixtures, or complex powder structures/mixtures. See Maglia, supra; Mayo, supra; and Poudel, supra; see also I.-W. Chen and X.-H. Wang, “Sintering dense nanocrystalline ceramics without final-stage grain growth,” Nature (2000) 404, pp. 168-171.
Known sintering techniques include pressureless sintering, hot pressing, hot isostatic pressing, spark plasma sintering, and high pressure sintering. However, when starting from nanostructured powders, hot pressing and hot isostatic pressing form materials with non-nanoscale microstructures having a size greater than 100 nm. Spark-plasma sintering can be used to produce nanostructures in bulk form; however, this technique doesn't allow the nanoparticles to retain their initial size and structure, and the nanostructure size is limited to greater than 60-70 nanometers. See Chaim, supra; see also Paris, supra. Shorter sintering times or lower temperatures can be used to retain the nanostructure, but in such cases residual porosity remains. Binders and/or sintering aids can be used in hard to sinter materials, but can result in residual impurities or changes in the chemistry/stoichiometry at particle boundaries. See Carsley, supra; see also Zhang, supra.
High pressure (2 GPa-8 GPa) sintering has recently been used to form nanocrystalline materials from nanopowder. However, residual porosity and impurity content remain in the final product as a result of incomplete powder processing techniques and/or exposure to contamination after powder processing. These process artifacts ultimately influence the properties of the bulk nanostructured material. See S.-C. Liao, Y.-J. Chen, B. H. Kear, and W. E. Mayo, “High pressure/low temperature sintering of nanocrystalline alumina,” Nanostructured Materials (1998) 10, pp. 1062-1079.
Thus, new processing techniques are required to create ceramic nanocomposites having no residual porosity and having nanoscale (less than 100 nm, more specifically less than 50 nm) ceramic constituents that are uniformly distributed and arranged in designated spatial order throughout the ceramic nanocomposite.